1. Field of the Invention
The present invention relates to a reticle used as a mask for forming a device pattern on a semiconductor wafer, at the photolithographic step in the production of a semiconductor. More particularly, the present invention relates to a reticle having a phase shifter capable of providing a fine pattern.
2. Description of the Related Art
According to the conventional phase shifting method, in the boundary area between a glass substrate of a reticle and a 180.degree. phase shifter the light intensity is reduced and an unnecessary pattern is formed. As a reticle overcoming this disadvantage, there is known a reticle having a multiple-stage phase shifter, whereby the phase shift quantity is stepwise changed, arranged between the glass substrate portion of the reticle and the 180.degree. phase shifter, as disclosed in 27P-ZG-6 of Collection of Preliminary Lectures at the '90 autumn meeting of Association of Applied Physics. FIG. 3a is a diagram illustrating the sectional structure of the reticle having this structure. In FIG. 3a, reference numeral 2 represents a reticle, reference numeral 7 represents a 180.degree. phase shifter, and reference numeral 8 represents a 90.degree. phase shifter. A reticle having a similar structure is disclosed in Japanese Unexamined Patent Publication No. 57-62052.
In the reticle having the above-mentioned structure, as shown in FIG. 3b, a reduction of the light intensity at the wafer during the light exposure can be controlled in the boundary area between the glass substrate and the 180.degree. phase shifter.
Nevertheless, the conventional multiple-stage phase shifter forming process is disadvantageous in that, since it is necessary to form a plurality of shifter films having a precisely controlled thickness, the reticle-preparing process becomes very complicated.